For fans of energy drinks, you may be getting more caffeine than the label claims, if the amount of caffeine is even listed on the label at all.

A US consumer group for Consumer Reports magazine tested 27 popular energy drinks — including Red Bull, Rockstar, and Starbucks Double Shot — finding that caffeine levels ranged from 6 milligrams to 242 milligrams per serving — which can be confusing for consumers.

Of the products tested, 11 didn’t list the amount of caffeine on the label, and among those 16 that did, five had more than 20% more caffeine than their label promised. One brand, Archer Farms Energy Drink Juice Infused, had about 70% less.

The highest level of caffeine was in the 5-hour Energy Extra Strength, which contained 242 milligrams. By comparison, Consumer Reports cites that an 8-ounce/236 mL cup of coffee has about 100 milligrams, while a 16-ounce/473 mL Starbucks Grande has about 330 milligrams.

While safe caffeine limits are still being studied, the reports states that current data suggests most healthy adults can safely consume up to 400 milligrams of caffeine per day; pregnant women, up to 200 milligrams; and children can consume up to 45 to 85 milligrams depending on weight. “An occasional energy drink is probably fine for most adults,” the researchers added.